As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among secondary batteries, research into lithium secondary batteries, which exhibit high energy density and operation potential and have long cycle life and a low self discharge rate and discharge voltage, is underway and such lithium secondary batteries are commercially available and widely used.
However, such lithium secondary batteries have problems in regard to safety and thus attempts are being made to address such problems.
In particular, when lithium secondary batteries are over-charged, excessive lithium is released from a positive electrode and excessive lithium is inserted into a negative electrode, whereby lithium metal having very high reactivity is precipitated on a surface of the negative electrode. Accordingly, the positive electrode also becomes thermally unstable, and rapid exothermic reaction occurs due to decomposition of an inorganic solvent used as an electrolyte, whereby a battery has problems in regard to safety, such as ignition, explosion, etc.
In addition, when a battery is penetrated by an object having electric conductivity such as a nail, electrochemical energy inside the battery is changed into thermal energy and rapid heat generation occurs. Accordingly, due to subsequent heat generation, a positive electrode or a negative electrode material is subjected to chemical reaction and thus rapid exothermic reaction occurs, whereby a battery has problems in regard to safety, such as ignition, explosion, etc.
When nail penetration, compaction, impact, or the like is applied, a positive electrode and a negative electrode in a battery are subject to partial short circuit. In this case, excessive current partially flows and thus heat generation occurs. Since the size of short circuit due to the partial short circuit is inversely proportional to resistance, short circuit current mainly flows at low resistance. Here, current flows through foil mainly used as a current collector, and, when a generated heat amount is calculated, very high heat is partially generates around a portion penetrated with a nail.
When heat is generated in a battery, a separator is contracted, additional short circuit in a positive electrode and a negative electrode is induced, and short circuit ranges are enlarged due to repeated heat generation and contraction of a separator. Accordingly, thermal runaway is generated, or a positive electrode, a negative electrode and an electrolyte included in a battery react together or combust. Since such a reaction is very high exothermic reaction, a battery finally ignites or explodes. Such risk increases with increasing capacity and energy density of a lithium secondary battery.
In order to address such a problem and enhance safety upon over-charging, attempts of adding an additive to a non-aqueous electrolyte have been made. However, securement of safety under conditions such as nail penetration, compaction, impact, etc. was not accomplished by adding an additive to a non-aqueous electrolyte.
There is an urgent need for technology to develop a secondary battery which may effectively enhance safety of a battery.